Non-aqueous Absorbents Research Articles

CO2 absorption and microwave regeneration with high‐concentration TETA nonaqueous absorbents

AbstractWhile nonaqueous organic‐amine absorbents have great potential in reducing energy consumption of CO2 capture, their absorption/desorption behaviors are still poorly understood when high‐concentration polyamines are used. On the other hand, large scale CO2 capture demands the use of high‐concentration polyamines absorbents. Therefore, we investigated the absorption and microwave regeneration behavior of high‐concentration triethylenetetramine (TETA) in three typical organic solvents, including polyethylene glycol 200 (PEG200), diethylene glycol (DEG), and ethylene glycol (EG). The results showed that high‐concentration TETA nonaqueous absorbents demonstrate advantages in absorption rate and regeneration energy consumption. The average absorption rate of 2.0 and 5.0 mol/L TETA/EG was 2.84 and 4.70 times that of 0.6 mol/L, respectively. The energy consumption of TETA/EG and TETA/DEG decreased by 29.4 and 25.6% as the concentration increased from 0.6 to 5.0 mol/L. In addition, the high‐concentration TETA/PEG200 absorbent demonstrated observable tolerance for water vapor, which usually exists in flue gas. Results also showed that microwave regeneration was superior to conventional conduction heating especially when high‐concentration TETA/PEG200 absorbent were used. The energy consumption of microwave regeneration of the absorbent with a concentration of 0.6, 2.0, and 5.0 mol/L was reduced by 65.9, 81.2, and 86.0%, respectively when compared with conduction heating. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.

Solubility of CO2 in nonaqueous system of 2-(butylamino)ethanol with 2-butoxyethanol: Experimental data and model representation

Nonaqueous amine-based system is an attractive solution to overcome high-energy-intensive CO2 capture process using the conventional aqueous amines. Advanced nonaqueous absorbent of 2-(butylamino)ethanol (BAE) with 2-butoxyethanol (2-BE) has been recently proposed for low-energy-consumption CO2 capture. In this work, Henry’s law constants of CO2 in the BAE/2-BE blend were obtained by N2O/CO2 analogy, and correlated in the temperature range of (283–333) K. Vapor-liquid equilibrium (VLE) data for the BAE + CO2 + 2-BE system at 65.4% (mass) BAE were also determined in a stirred equilibrium cell at temperatures of (313–393) K and CO2 partial pressures up to 275 kPa. A single apparent equilibrium constant KCO2,app was proposed for this system and correlated as a function of temperature, carbonated degree of amine and CO2 loading. Solubility data were well represented by the modified Kent-Eisenberg model with an average absolute relative deviation (AARD) of 13%.

Densities and viscosities of partially carbonated nonaqueous blends of 2-(methylamino)ethanol with 2-alkoxyethanols at temperatures of (293.15 to 353.15) K

Nonaqueous blends of alkanolamines with 2-alkoxyethanols have recently attracted much attention for an energy-efficient CO2 capture process. In this work, densities and viscosities of partially CO2-loaded mixtures of 2- (methylamino)ethanol (MAE) with 2-methoxyethanol (EGME), 2-ethoxyethanol (EGEE) and 2-butoxyethanol (EGBE) were measured at atmospheric pressure over the temperature range of (293.15 to 353.15) K with amine mass fractions of (0.10, 0.30 0.40 and 0.50) containing various CO2 loading. It was observed that both densities and viscosities of the carbonated solutions showed noticeable changes as increasing CO2 loading, amine concentration and temperature. Dimensionless parameters from the CO2-free and CO2-loaded property data were fitted by proposed correlations. The calculated values of density and viscosity from the correlation models matched well with the experimental results of MAE + CO2 + 2-alkoxyethanols, with AARDs within 0.28% and 2.20% for density and viscosity respectively. These results are useful to evaluate the mass transfer and hydrodynamic performance in a CO2 capture process using these nonaqueous absorbents.

Carbon dioxide capture by non-aqueous blend in rotating packed bed reactor: Absorption and desorption investigation

Carbon dioxide (CO2) capture and storage (CCS) is currently the most effective technology to reduce CO2 emissions, but it has crucial issues of high energy consumption and low mass transfer efficiency in capture process. In this work, 2-amino-2-methyl-1-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEEA), N-methyl pyrrolidone (NMP) tri-solvent blend is used as a novel non-aqueous absorbent for CO2 capture process in rotating packed bed (RPB) reactor, which is employed to intensify the mass transfer in both absorption and desorption process. The experimental results show that this new method has good absorption performance. When the CO2 loading of the lean absorbent is 0.035 mol CO2/mol amine, the CO2 capture efficiency is still 89% and the overall volumetric mass transfer coefficient (KGa) is 2.67 kmol·m−3·h−1·kPa−1. For desorption process, this study adopted the direct steam stripping (DSS) technique to reduce energy consumption. The optimal regeneration energy consumption by DSS of this non-aqueous absorbent in RPB is about 2.46 GJ/ton CO2, which is 36.6% lower than that in conventional reboiler regeneration process with 30% MEA solution. The results indicate that this new method has the advantages of high efficiency and low energy consumption, exhibiting good industrial application prospect.

Performance and mechanism of CO2 absorption in 2-ethylhexan-1-amine + glyme non-aqueous solutions

In this work, novel non-aqueous absorbents composed of 2-ethylhexan-1-amine (EHA) and glyme were proposed for CO2 capture. The absorption performance of CO2 in EHA + diglyme, EHA + triglyme and EHA + tetraglyme non-aqueous solutions was investigated and the viscosities (η) of the CO2-saturated absorbents were measured. Besides the experiments, kinetic models were applied to correlate the CO2 absorption. The activation energy (Ea) was obtained from Arrhenius equation, and the absorption mechanism was deduced. The results showed that both Lagergren model and Avrami model can accurately correlate and predict the time-dependent absorption amount, thus an optimized composition under which excellent absorption performance and relatively low η and Ea can be simultaneously achieved was determined. Compared to water-based absorbents like MEA, the optimized non-aqueous absorbents take the advantages of better absorption performance and lower activation energy. Moreover, the glyme solvents have about 50% lower specific heat capacities and much higher boiling points (≥435 K) than water, which is expected to greatly reduce the sensible heat and the latent heat of the solvent during regeneration. Therefore, the proposed novel absorbents have promising industrial application potential in the CO2 capture process.

A screening study of alcohol solvents for alkanolamine-based CO2 capture

The present study was aimed at developing a non-aqueous absorbent system that does not form any precipitate/solid upon exposure to CO2. More specifically, the non-aqueous absorbent system must be a homogeneous mixture (single phase) within the whole range of CO2 loading. At the same time, sufficient amine regeneration must occur at a low temperature of 80–90 °C compared to reference non-aqueous monoethanolamine (MEA)-based absorbents. To achieve this target, 18 alcohols were investigated as solvents and 4 alkyl-linked alkanolamines were evaluated as absorbents. Several experiments mixing alkanolamines with alcohols were performed to evaluate their CO2 capture performance. High-performance non-aqueous liquid absorbents (without solid/precipitate formation) with the advantages of higher absorption rates, higher cyclic capacities, higher regeneration efficiencies, lower specific heats, and lower heats of reaction compared to reference MEA-based non-aqueous absorbents were successfully developed herein. The favorable features of these absorbents will provide the basis for the discovery of potential non-aqueous amine-based absorbents.

2-Amino-2-methyl-1-propanol based non-aqueous absorbent for energy-efficient and non-corrosive carbon dioxide capture

The large-scale deployment of carbon dioxide (CO2) capture using aqueous amines is mainly limited by its intensive energy penalty. In this regard, non-aqueous amine solutions have high energy-saving potential because organics have lower heat capacity and vaporization enthalpy than water. In this study, 2-amino-2-methyl-1-propanol (AMP) coupled with activators in an inert organic solvent (N-methyl pyrrolidone, NMP) is proposed for energy-efficient CO2 capture. The relationships between activator properties and CO2 capture performance, such as absorption capacity, regeneration efficiency, and corrosion behavior, were investigated. The results showed that the non-aqueous AMP-AEEA (2-(2-aminoethylamino)ethanol)-NMP solution not only possessed high CO2 capacity (1.65 mol·kg−1 solution) but also retained nearly 90% of its initial CO2 capacity after the 4th cycle of regeneration. Moreover, it presented a non-corrosive behavior after saturated absorption, clearly showing its superiority over the benchmark monoethanolamine (MEA) solution. The 13C nuclear magnetic resonance (NMR) spectra provided evidence of CO2 reacting with AMP-AEEA in NMP to form carbamates, which could be easily regenerated under thermal desorption. The specific solvent loss was 0.14 kg⋅kg−1 CO2 and the total heat duty of AMP-AEEA-NMP solution was only about half that of the MEA solution, which can be attributed to the absence of water and properties of the inert organic solvent. With the perfect CO2 capture performance, non-corrosive behavior, and significant reduction of energy consumption, the novel solution is a promising candidate for CO2 capture.

Novel Amino-Functionalized Ionic Liquid/Organic Solvent with Low Viscosity for CO2 Capture.

To achieve low regeneration energy consumption and viscosity, a novel amino-functionalized ionic liquid [TEPAH][2-MI] combined with organic solvents has been proposed for CO2 capture in this work. The results demonstrated that the absorption loading of [TEPAH][2-MI]/N-propanol (NPA)/ethylene glycol (EG) was 1.72 mol·mol-1 (28 wt %, 257 g·L-1), which was much higher than that of monoethanolamine/water, and the regeneration efficiency was maintained at 90.7% after the fifth regeneration cycle. The viscosities of the solution were only 3.66 and 7.65 mPa·s before and after absorption, respectively, which were significantly lower than those of traditional nonaqueous absorbents. The reaction mechanism investigated via 13C NMR and quantum calculations summarized that CO2 first reacted with the amino group of [TEPAH]+ to form the carbamates through the zwitterion formation and protonation process, while CO2 reacted with the N atom of [2-MI]- to directly form the carbamate. Then, some of them further reacted with NPA and EG to form the carbonates. Moreover, Nπ and Nτ tautomers of [TEPAH][2-MI] could convert into each other continuously when CO2 was absorbed. During CO2 desorption, the carbamates and carbonates reacted with AFILH+ to decompose and released CO2 directly.

Viscosity of 2-ethylhexan-1-amine (EHA)-diglyme, EHA-triglyme and EHA-tetraglyme non-aqueous solutions and its effect on initial absorption rate

The viscosities (η) of 2-ethylhexan-1-amine (EHA), diglyme, triglyme, tetraglyme and EHA-diglyme/triglyme/tetraglyme non-aqueous solutions were measured by using the NDJ-5S rotational viscometer at 101 kPa. The temperatures (T) ranged from 303.2 K to 323.2 K. The mass fractions of EHA (wEHA) ranged from 0.100 to 0.900. The viscosity was modeled and the calculated results agreed well with the experiments. The effects of wEHA and T on viscosity were demonstrated on the basis of experiments and calculations. The initial absorption rates of CO2 (r) in EHA-diglyme/triglyme/tetraglyme non-aqueous solutions were measured, and the effects of viscosity, absorbent concentration and absorption rate constant of CO2 (k) on initial absorption rates were demonstrated. Our research shows that in the temperature range of 303.2 K to 323.2 K, the proposed three non-aqueous absorbents have small solution viscosities (<3 mPa·s), which is beneficial to the mass transfer of CO2 absorption and the initial absorption rate. Thus the proposed non-aqueous absorbents have good application potential in the field of CO2 capture process.

Development of High-Capacity and Water-Lean CO2 Absorbents by a Concise Molecular Design Strategy through Viscosity Control.

The exponentially increasing viscosity of water-lean CO2 absorbents during carbon capture processes is a critical problem for practical application, owing to its strong correlation with systems' mass transfer properties, as well as convenience of transportation. In this work, a concise strategy based on structure-viscosity relationships is proposed and applied to construct a series of functionalized ethylenediamines as single-component absorbents for post-combustion CO2 capture. These nonaqueous absorbents have outstanding viscosities (50-200 cP, 25 °C) at their maximal CO2 capacities (up to 22 wt % or 4.92 mol kg-1 , 1 bar), and are readily regenerated at low temperatures (50-80 °C) under ambient pressure. Additional capture of CO2 through physisorption could also be achieved by operating at high pressures. The CO2 capture and release process is systematically investigated by means of 13 C NMR spectroscopy, differential scanning calorimetry (DSC), in situ FTIR analysis, and density functional theory (DFT) calculations, which could provide sufficient spectroscopic details to reveal the ease of reversibility and enable rational interpretation of the absorption mechanism.

Nonaqueous amine-based absorbents for energy efficient CO2 capture

High energy consumption for CO2 capture using aqueous amines is still a major drawback limiting its deployment on a global scale. In this work, the absorption-desorption performance of CO2 into several blends of monoethanolamine (MEA) or diethanolamine (DEA) with glycol ethers (2-methoxyethanol (2ME) and 2-ethoxyethanol (2EE)) as nonaqueous solvents has been investigated at ambient pressure. Reaction mechanisms and the relative heat duty for these nonaqueous systems were also explored and compared with those of the aqueous MEA. Results showed that the nonaqueous absorbents with 5.0 M MEA had comparable absorption capacity with aqueous 5.0 M MEA and higher desorption efficiency resulting in a larger cyclic capacity (1.45 mol kg−1). Higher partial pressure ratio of CO2 to solvent vapor under the desorption conditions was also observed for these nonaqueous systems. 13C NMR, FTIR spectra and online conductivity analysis provided evidences that CO2 can react with MEA in nonaqueous solvents forming the ionic couples (i.e. protonated MEA and carbamate) as well as carbamic acid at high CO2 loading. The mixture of MEA and 2ME or 2EE could significantly reduce the energy consumption by about 55% as compared to the benchmark aqueous 5.0 M MEA system.

Performance and Mechanisms of Triethylene Tetramine (TETA) and 2-Amino-2-methyl-1-propanol (AMP) in Aqueous and Nonaqueous Solutions for CO2 Capture

Nonaqueous amines are currently being explored for energy-efficient postcombustion CO2 capture, but the absorption capacity and regeneration efficiency of most absorbents still need to be further improved. In this work, a mixture of triethylene tetramine (TETA) and 2-amino-2-methyl-1-propanol (AMP) dissolved in ethanol was chosen as a nonaqueous absorbent for CO2 capture and performed with high absorption loading and regeneration efficiency. In addition, the changes of the components during the absorption–desorption process and reaction mechanisms of nonaqueous absorbents were explored by 13C NMR spectroscopy and compared with those of aqueous absorbents. The results showed that TETA/AMP/ethanol had higher absorption loading and regeneration efficiency (3.71 mol kg–1, 95.4%) after the fifth regeneration cycle than TETA/AMP/water (3.54 mol kg–1, 38.8%) when the amine mass concentration was 40 wt %. CO2 capture by TETA/AMP aqueous and nonaqueous solutions proceeded by two different mechanisms. Polycarbamate...

Characterization and comparison of the CO2 absorption performance into aqueous, quasi-aqueous and non-aqueous MEA solutions

Herein, we investigated quasi-aqueous and non-aqueous absorbents based on organic solvents to reduce the amount of water used in wet CO2 capture by amine- and water-based sorbents to <50%. Solvent optimization was performed through screening and vapor pressure tests, and the performances of quasi-aqueous and non-aqueous sorbents prepared using selected organic solvents were evaluated in terms of CO2 absorption rates and equilibrium absorption capacities. All absorbents were subjected to thermogravimetric analyses before and after CO2 absorption to determine their thermal stabilities. Moreover, the prepared absorbents were characterized in terms of reaction heat, sensible heat, and latent heat of evaporation, which were used to determine their regeneration energies. The obtained results revealed that quasi-aqueous/non-aqueous absorbents are superior to conventional water-based ones, exhibiting maximized CO2 absorption and lower regeneration energies.

High regeneration efficiency and low viscosity of CO2 capture in a switchable ionic liquid activated by 2-amino-2-methyl-1-propanol

Switchable ionic liquids (SILs) are novel green non-aqueous absorbents for energy-efficient post-combustion CO2 capture. However, their regeneration efficiency and viscosity still need to be improved. SILs can be activated by some alkylol amines to improve their regeneration efficiency and reduce viscosity. In this work, mixtures of the SIL 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)/ethanol and 2-amino-2-methyl-1-propanol (AMP) were used for CO2 capture. The constitution of the absorbent was optimized. Absorption and regeneration were investigated, and the detailed reaction mechanisms for CO2 capture were analyzed using 13C NMR and FTIR. The results demonstrated that CO2 could be absorbed to form carbonates in two ways. On the one hand, CO2 reacted with DBU and ethanol to directly form carbonates. On the other hand, CO2 initially reacted with AMP to form carbamates, and then, the carbamates reacted with ethanol to form carbonates. The CO2 loading of the optimized absorbent was 2.98molkg−1. The saturated absorbent began to desorb at 80°C, and the regeneration efficiency reached 99.3% when the temperature rose to 120°C. After five cycles, the regeneration efficiency remained over 90%. Compared to DBU/ethanol, the regeneration efficiency of the DBU/AMP/ethanol mixtures was significantly better, while the viscosity was greatly reduced, which is beneficial for the industrial application.

Energy-efficient CO2 capture using potassium prolinate/ethanol solution as a phase-changing absorbent

High energy consumption for aqueous amine-based CO2 capture process is still a major drawback limiting its commercial deployment. Most of heat is used for water vaporization during solvent regeneration for CO2 stripping. Development of phase-changing solvents is an attractive energy-efficient method to reduce energy requirement. In this work, we have proposed potassium prolinate (ProK)/ethanol solution as a novel non-aqueous absorbent. Its physical properties and the rate of CO2 absorption were investigated and compared with the aqueous ProK systems. Phase behaviours during CO2 absorption and partition of CO2 absorbed between liquid and solid phases were characterized. The main species in solid precipitates were identified as bicarbonate and proline salts by 13C NMR and XRD. Moreover, the regenerated absorbents can remain relatively stable after absorption-desorption cycles.

CO2 capture by piperazine mixed with non-aqueous solvent diethylene glycol in a rotating packed bed

In this study, a non-aqueous absorbent, 40.8wt% piperazine (PZ)/59.2wt% diethylene glycol (DEG), was proposed to capture CO2 in a rotating packed bed (RPB). It was observed that the 40.8wt% PZ could dissolve into DEG without precipitation at 20°C and regeneration energy could be reduced because the heat capacity of DEG is lower than that of water. Although the viscosity of 40.8wt% PZ/59.2wt% DEG was 7 times as much as that of 40.8wt% PZ/59.2wt% H2O, the difference in CO2 capture efficiency between these two absorbents in RPB was only 7.95% under the operation condition to satisfy 90% CO2 capture efficiency. At the rotating speed of 1600rpm, the heights of transfer unit of the proposed non-aqueous absorbent were all less than 6cm, indicating the potential of volume reduction of RPB with respect to the conventional packed column even for high viscosity absorbent. The results obtained from 23 factorial design showed that the most significant operational variable of CO2 capture efficiency for 40.8wt% PZ/59.2wt% DEG in RPB was gas flow rate, followed by liquid flow rate and temperature. Finally, the most appropriate operation conditions to achieve CO2 capture efficiency exceeding 90% were found out by path of the steepest ascent.